# What Is The Probability of Having Fraternal Twins on Christmas Day ?

#### Qcan

##### New member
Considering that the least amount of births in the Western World happen on December 25th, I would think the odds are a little long.

#### Dr.Peterson

##### Elite Member
Considering that the least amount of births in the Western World happen on December 25th, I would think the odds are a little long.
I guess you just have to gather some data and find out. Like your other question, this is for statisticians and census takers, not for mathematicians. But I would expect that P(fraternal twins | birth on Dec. 25) would be about the same as P(fraternal twins), so you can probably just multiply the two probabilities (for fraternal twins and for Christmas births) together.

But where did you get your information about number of births?

#### Qcan

##### New member
Common knowledge. The same applies but to a lesser degree to July 4th, December 31st and a few other. This is primariliy due to postponed inducments etc. The same applies to certain days of the week. This has been the case over the past 100 years or so in urban area. On a side not, the odds of fraternal twins is approximatly 0.35 %.

I am asking because I have cousins that are twins that were born on Xmas day. I thought someone might have know the answer instead of going through the calculation.

I assume (if my math is correct), that without getting precise and assuming that all days had the same amount of births, that the answer would be .35 * .2740 = .0959 or 96 in every 100,000 births.

Data set is from Fivethirtyeight - https://github.com/fivethirtyeight/data/tree/master/births

#### Dr.Peterson

##### Elite Member
Common knowledge. The same applies but to a lesser degree to July 4th, December 31st and a few other. This is primariliy due to postponed inducments etc. The same applies to certain days of the week. This has been the case over the past 100 years or so in urban area. On a side not, the odds of fraternal twins is approximatly 0.35 %.

I am asking because I have cousins that are twins that were born on Xmas day. I thought someone might have know the answer instead of going through the calculation.

I assume (if my math is correct), that without getting precise and assuming that all days had the same amount of births, that the answer would be .35 * .2740 = .0959 or 96 in every 100,000 births.

Data set is from Fivethirtyeight - https://github.com/fivethirtyeight/data/tree/master/births
I'm not sure of your calculation; did you intentionally not convert percentages to decimals? I find that 0.2740% ignores the date; I'd expect you to have used the probability of a Christmas birth.

Taking your value of 0.35% of births being fraternal twins (which seems low to me, but I know it varies a lot), and the claim here that the number of births on Christmas is 0.57 of the 1/365 that would be expected, I find that the fraction of births that are fraternal twins on Christmas is 0.0035*0.57*1/365 = 5.466e-6, which means 0.55 per 100,000 births.

#### Qcan

##### New member
You are correct.... I should have wrote 3.7 % (Big oops). Thanks for pointing that out.